Cell Communication: AP Biology Unit 4

Direct Contact Signaling

Contact through cell junctions. Gap junctions in animals, or plasmodesmata in plants.

Cell to Cell Recognition

Another type of direct local signaling. Glycoproteins-we see this in the immune system.

Paracrine Signaling

Secreting cells discharge local regulator molecules to nearby target cells. Like growth factors to tell a cell to grow and divide.

Synaptic Signaling

Nerve cells release NT across a synapse, stimulating the target cell.

Long Distance Cell Communication-Hormones

-Specialized cells called endocrine cells secrete hormones into vessels. Hormones travel through the blood stream to target cells in the body.

-Plant hormones can move through vessels or through cells or even through the air as gas

Signal Transduction Pathway

Series of steps/reactions in a cell that are caused by a signal on the cell’s surface that triggers a specific cellular response.

3 Stages of Cell Signaling

1. When a cell encounters a signaling molecules it must be recognized by a receptor protein

2. The signal must be changed or transduced into another form inside the cell

3. The cell can respond. Signaling molecule=ligand

Ligand

A molecules that specifically bonds to another molecule (usually a larger one) and usually results in a shape change to the receptor protein.

Reception: Stage 1 of Cell Signaling

Signaling molecules from outside the cell bonds to a receptor protein.

1. The binding between a ligand and a receptor is highly specific. A change in the receptor protein is the first step in the transmission of a signal.

2. Receptors are found in two places

Intracellular Receptors

Intracellular receptors are found inside the plasma membrane in a cytoplasm or nucleus. Hydrophobic ligands would cross the membrane and bind to an intracellular receptor.

Ex. testosterone

Cell Surface Receptors

Bind to water soluble ligands.

Ex. epinephrine binds to surface liver cells receptors causing a cascade that leads to the conversion of glycogen into glucose.

G-Protein Coupled Receptors

Also known as GCPR’s

1. ligand binds to receptor causing a shape change in G-protein coupled receptor

2. Receptor is activated and binds to an activated G protein

3. Activated G protein binds to an enzyme causing ONE cellular response

4. Return to original shape for reuse

Ligand Gated Ion Channel Receptors

1. Ion channel is closed without ligand present

2. Binding of the ligand to the receptor on the ion channel opens the gate for ions to flow through from high to low concentration

3. Causes a change in concentration gradient leading to a cell response

4. Removal of the ligand causes the gate to close, prevents ions from moving, cellular response stops.

ex. nervous system

Transduction-Step 2

A step(s) to bring about a specific cell response like a shape change of a protein or gene activation.

1. usually more than one step-cascade

2. can amplify a signal and help the cell regulate it’s processes

3. protein kinase: Inactive protein kinases are phosphorylated and activated with the help of the previously activated protein. Ultimately the final protein goes through a shape change as a result.

4. protein phosophatase

Protein Kinase

An enzyme that transfers a P group from ATP to an inactive protein. Phosphorlates this protein.

Protein Phosphatase

Enzyme that removes a P group (dephosphorylate) from the active protein to turn off the signal tranduction pathyway.

Second Messengers

Small, non-protein, water-soluable molecules or ions that move through diffusion unlike an extracellular ligand attaching to the membrane receptor, (first messanger).

ex. cAMP and calcium ions

Example of a Second Messenger: cAMP from epinephrine signal

-cAMP comes from ATP breaking into 2 ATP and cAMP

-Triggered by epinephrine (first Messenger, ligand)

-G protein activated adenylyl cyclase

-Adenylyl cyclase catalyzes the reaction of ATP to cAMP and 2 P

-cAMP can broadcast to entries cytoplasm and activate Protein Kinase A

--Will continue to make more cAMP and activate more Protein Kinase A until inhibitors are activated